1. Field of the Invention
This invention relates to a hard-type endoscope apparatus having an observation optical system located in its insertion portion for transmitting an image to be observed to the side of its eye-piece portion via a plurality of lenses.
2. Description of the Related Art
In general, an observation optical system incorporated in an endoscope, in particular, in a hard type endoscope includes an objective system, a relay system and an eye-piece system, and comprises several tens of glass lenses. An image to be observed is transmitted to the side of the eye-piece system via those lenses.
FIG. 8 shows a prior art arrangement of glass lenses a employed in a general hard-type endoscope. The glass lenses a are generally inserted in a metal pipe (scope barrel) b so as to keep the optical axes of the lenses in line. Spacer pipes c are interposed between each adjacent pair of the lenses a for keeping them at regular intervals.
In many conventional cases, a metal pipe is used as the spacer pipe c. At the time of observation, a light beam passes through the spacer pipes c for enabling an image to be observed. Further, as is shown in FIG. 8, light beams L.sub.1 and L.sub.2 passing out of the field of view are reflected from inner surface portions of the spacer pipes c.
Since in the above-described general hard-type endoscope, the distance between each adjacent pair of the lenses a is relatively short, the reflected light beams L.sub.1 and L.sub.2 soon enter outer peripheral portions of the lenses a. The outer periphery of each lens, in general, is not completely polished, and has a relatively rough surface. Therefore, each of the reflected light beams L.sub.1 and L.sub.2 change to scattered light beams L.sub.3 at the periphery of the lens and disappear. This being so, only subjecting the lens a to a reflection-preventing treatment can prevent the occurrence of flare in a picked-up image, without subjecting the inner surface of the spacer pipe c to any special reflection-preventing treatment.
Further, there is a conventional endoscope, in which the inner surface of the metal spacer pipe c is coated with a matting black film so as to prevent a light beam passing through the pipe for image observation, from being reflected from its inner surface.
On the other hand, attention has been paid to a disposable endoscope, and various improvements have been developed for the same. In this case, the reduction of the manufacturing cost is regarded as important. To this end, an optical component such as a lens, a light guide, etc. has been made of a resin, as is disclosed in PCT W093/17362.
Moreover, Japan Patent Application KOKAI Publication No. 61-47913 discloses a technique for forming the barrel of a zoom lens for video cameras of a resin in order to reduce its manufacturing cost. DE 3912720 discloses a technique for positioning a relay lens system by means of a thermally contractive tube. Furthermore, there are known a technique for integrally forming a resin lens and a space pipe as one body, and a technique for providing, in a pipe, flange means to define the distance between each adjacent pair of lenses aligned in the pipe.
It is considered to reduce the number of the lenses a of an observation system employed in a disposable hard-type endoscope so as to reduce the manufacturing cost of the endoscope, as compared with the conventional one equipped with several tens of lenses a.
In this case, however, the distance between each adjacent pair of the lenses a inevitably increases as in the endoscope disclosed in PCT W093/17362, and therefore relatively long spacer pipes c are necessary. In the case of using relatively long spacer pipes c, it is possible that scattered light, which is part of light indicative of an image to be observed, reflects from the inner periphery of the long spacer pipes c, and the reflection light is transmitted to the eye piece, thereby causing flare in the image.
In order to avoid the occurrence of flare, a black matting layer of black chrome, etc. is generally formed on the inner periphery of the spacer pipe c to prevent reflection of light therefrom. However, it is difficult to uniformly coat the overall inner periphery of the pipe with the black matting layer, since the spacer pipe is a thin pipe having its axial length formed much longer than its radial length.
Further, Japan Patent Application KOKAI Publication No. 61-64421 discloses a technique for forming the lens barrel of a camera of a resin in order to reduce its manufacturing cost. In this case, however, the lens barrel consists of a plurality of separate barrel elements, and hence is more expensive than a lens barrel which consists of a single barrel element. In addition, although the technique for coating the inner periphery of a spacer pipe with a black matting layer is applicable, for example, to a camera having a relatively large diameter, it is hard to apply to an endoscope having an inserting portion whose diameter is limited to a low value. Moreover, in the case of forming an axially-long thin pipe by injection molding of a synthetic resin, it is necessary to design a draft, etc. in a die, which makes it difficult to form the die.
As in the case of DE3912720 where the relay lens system is positioned by means of a thermally contractive tube, various improvements have been made to the spacer pipe c to reduce the manufacturing cost. However, no improvement has been made to prevent reflection of light from the inner periphery of the spacer pipe c thereby to prevent occurrence of flare in an image to be observed.